A number of compositions are known which catalyze the oxidation of carbon monoxide to carbon dioxide at ambient temperatures. The effectiveness of these catalyst compositions is, however, generally limited to oxidation of carbon monoxide from relatively dry gas streams which are free from sulfur dioxide.
A commonly used catalyst composition for oxidation of carbon monoxide is a mixture of palladium and copper chlorides, PdCl.sub.2 -CuCl.sub.2, supported on activated carbon or alumina, for example. This catalyst composition promotes carbon monoxide oxidation from moist air streams at room temperature with exceptional efficiency and for virtually an indefinite period of time. However, if the air stream contains sulfur dioxide, even to the extent of a few parts per million by volume thereof, the catalyst composition is quickly poisoned and will have only a very short useful life. Moreover, exposure to sulfur dioxide causes irreversible damage to the catalyst, as evidenced by the fact that degradation of the catalyst activity continues even after removal of the sulfur dioxide from the gaseous stream which the catalyst is being used to treat.
The precise reason for the rapid loss of catalytic oxidation activity of a catalyst such as PdCl.sub.2 -CuCl.sub.2 supported on activated carbon in the presence of small amounts of sulfur dioxide is not known. It has been recognized that the phenomenon of catalyst poisoning, that is, the ability of trace amounts of some impurity to destroy the efficiency of a catalyst, is a consequence of the fact that catalytic activity is often concentrated on a relatively small portion of the surface of a catalyst. These areas of catalytic activity, called active centers, are thought to preferentially adsorb the poisoning material. When the active area of the catalyst is small, it can be completely covered by small amounts of the poison, in this case, sulfur dioxide. On the other hand, activated carbon itself is known to catalyze the oxidation of sulfur dioxide to sulfur trioxide, and the latter may further react with moisture present on the activated carbon to form sulfuric acid. Thus, the pores of the activated carbon may become occluded by adsorption of either or both of sulfur trioxide and sulfuric acid. Such occlusion of the activated carbon pores may result in covering of the catalyst composition impregnated thereon. Also, the catalyst composition may be subject to attack by the sulfur trioxide or the sulfuric acid which has been formed. On the other hand, it has been found that when sulfuric acid is added onto the catalyst directly, that the catalytic efficiency of the activated carbon supported catalyst is not affected.
The present invention provides a method for overcoming this problem of catalyst inefficiency or poisoning produced by sulfur dioxide, and comprises a method of treating the activated carbon support for the catalyst so as to greatly improve the performance of the catalyst in oxidation of carbon monoxide, in the presence of sulfur dioxide. Particularly, the present invention provides a method for oxidative modification of the activated carbon support, and impregnation thereof with an inert hydrophobic compound, as well as the thus treated support together with an oxidation catalyst impregnated thereon.
It is known in the art that an activated carbon supported Cd.sup.2+ catalyst employed in acetaldehyde synthesis can be pretreated with nitric acid to give improved performance. See Siedlewski et al., Chem. Stosow. (1973) 19(2), 221-8 (Chemical Abstracts, Vol. 80, 412412). However, Cd is not a catalyst for carbon monoxide oxidation as used in the present invention.